The US military has the following missions, by law and treaty:
1. Protect the United States
2. Provide mutual defense to Canada, the entire continent of Europe, and South Korea.
3. Defend counties like Japan and Iceland, which are treaty-bound not to have militaries of their own.
By tradition and policy, but not necessarily binding treaty or law, the US military also has the following missions:
4. Protect the entire Western Hemisphere from foreign aggression (the Monroe Doctrine).
5. Protect the economic supply chain of the developed world, particularly the oil supply.
6. Occasionally intervene, militarily, in humanitarian crises (this goes back and forth; yes in Somalia, no in Rwanda, yes in Yugoslavia).
7. Provide purely humanitarian assistance in case of natural disasters (the military has tons of logistical ability that's occasionally repurposed to provide food and water to e.g. Haiti whenever it gets hit by a hurricane).
8. Keep the world's sea lanes open for shipping (this is kind of a shared responsibility with every major naval power, but we're practically the only one left).
Also, there are non-functional requirements to the US military, driven by purely political forces that almost no potential US enemy has:
9. Minimize friendly casualties. Spare no expense to accomplish this.
10. Minimize civilian casualties.
11. Maintain good PR so they don't look like bullies.
In other words, the US has to bear the bulk of the entire world's military requirements, and not just be able to win wars, but win them in absolute routs that are nearly unprecedented in world history. If anything, the US military is undersized for its current requirements. It's oversized if you're looking at requirement #1, but that's a ridiculous understatement of the actual requirements, or any realistic set of requirements, for the US military.
1. Protect the United States
2. Provide mutual defense to Canada, the entire continent of Europe, and South Korea.
3. Defend counties like Japan and Iceland, which are treaty-bound not to have militaries of their own.
By tradition and policy, but not necessarily binding treaty or law, the US military also has the following missions:
4. Protect the entire Western Hemisphere from foreign aggression (the Monroe Doctrine).
5. Protect the economic supply chain of the developed world, particularly the oil supply.
6. Occasionally intervene, militarily, in humanitarian crises (this goes back and forth; yes in Somalia, no in Rwanda, yes in Yugoslavia).
7. Provide purely humanitarian assistance in case of natural disasters (the military has tons of logistical ability that's occasionally repurposed to provide food and water to e.g. Haiti whenever it gets hit by a hurricane).
8. Keep the world's sea lanes open for shipping (this is kind of a shared responsibility with every major naval power, but we're practically the only one left).
Also, there are non-functional requirements to the US military, driven by purely political forces that almost no potential US enemy has:
9. Minimize friendly casualties. Spare no expense to accomplish this.
10. Minimize civilian casualties.
11. Maintain good PR so they don't look like bullies.
In other words, the US has to bear the bulk of the entire world's military requirements, and not just be able to win wars, but win them in absolute routs that are nearly unprecedented in world history. If anything, the US military is undersized for its current requirements. It's oversized if you're looking at requirement #1, but that's a ridiculous understatement of the actual requirements, or any realistic set of requirements, for the US military.